1. Field of the Invention
The present invention relates to a multiplex voltage measurement apparatus, and specifically to a multiplex voltage measurement apparatus for measuring a voltage of each of serially connected N voltage sources.
2. Description of the Related Art
A high-power electric source of several hundred voltages for an electric vehicle is formed by a number of secondary battery cells, such as nickel-hydrogen storage cells, which are serially connected to each other. Each of the serially connected battery cells should be monitored for its capacity for the purpose of charge/discharge control.
In particular, a battery formed by 240 serially connected cells produces a total voltage of 288 V. In such a battery, it is physically difficult to monitor each cell. In Japanese Laid-Open Publication No. 8-140204, for example, the voltage is measured for each of 24 modules each including 10 cells.
In an electric vehicle, high-voltage systems are electrically insulated from a chassis in order to avoid hazardous conditions. On the other hand, since a processor for charge/discharge control uses a potential of the chassis as a reference potential, the voltage of a battery should be insulatively measured.
In the battery disclosed in Japanese Laid-Open Publication No. 8-140204, an insulation circuit unit including an operational amplifier, an AD converter, a photocoupler, a power supply, etc., is provided for each module. The structure of such a battery is enormously complicated.
As means of insulatively measuring the output voltage of a sensor or the like, a flying capacitor is known. FIG. 3 shows a structure of a multiplex voltage measurement apparatus 300. In this example, the number of voltage sources (N) is 5.
Serially-connected voltage sources V1-V5 are connected to a capacitor 3 through voltage detection terminals T1-T6, and through a first sample switch 1 formed by switches S1, S3, and S5 and a second sample switch 2 formed by switches S2, S4, and S6. The capacitor 3 is connected to a voltage measurement circuit 5 through a third sample switch 4 formed by switches 4a and 4b. 
FIG. 4 is a timing chart for opening/closure of the respective switches S1-S6, and 4a and 4b. An operation of the multiplex voltage measurement apparatus 300 is now described with reference to FIG. 4 in conjunction with FIG. 3.
Prior to measuring the voltages of the voltage sources V1-V5, the switches S1-S6, and 4a and 4b are all opened (OFF). During period P1, first of all, the switches S1 and S2 are closed (ON), whereby the voltage of the voltage source V1 is applied to the capacitor 3, and a charge is stored in the capacitor 3. After being kept closed (ON) for a predetermined time period, the switches S1 and S2 are turned off. Then, after a predetermined time has elapsed since the switches S1 and S2 were turned off, the third sample switch 4 (switches 4a and 4b) is turned on, whereby the charged voltage in the capacitor 3, i.e., the voltage of the voltage source V1, is transferred to the voltage measurement circuit 5.
As a matter of course, a driving circuit of each switch and a contact point of the switch are kept separated. The first sample switch 1 is not closed while the third sample switch 4 is closed, and the second sample switch 2 is not closed while the third sample switch 4 is closed. Therefore, the voltage of the voltage source V1 is insulatively measured, i.e., when the voltage of the voltage source V1 is measured, the voltage source V1 and the capacitor 3 are insulated.
During period P2, the switches S2 and S3 and the switches 4a and 4b are similarly turned on and off, and during period P3, the switches S3 and S4 and the switches 4a and 4b are similarly turned on and off. In this way, as shown in FIG. 4, the multiplex voltage measurement apparatus 300 operates in a multiplex manner to measure the voltage values of the voltage sources V1-V5.
In the above structure of the conventional voltage measurement apparatus, when measuring the voltage source V1 after the voltage source V5 has been measured, the switches S1 and S2 are closed (ON). However, if one of the switches S1 and S2 is out of order, e.g., one of the switches S1 and S2 which should be closed is left opened, the voltage of the voltage source V1 cannot be stored in the capacitor 3, and the charge in the capacitor 3 which was stored when the voltage source V5 was measured remains in the capacitor 3. Since the polarity of the capacitor 3 when the voltage of the voltage source V1 is measured is the same as that when the voltage of the voltage source V5 is measured, the voltage measurement apparatus 300 erroneously reads the voltage of the voltage source V5.
Thus, when one of the switches S1 and S2 is out of order so that it cannot be closed, the previously stored charge is left in the capacitor 3. Therefore, the voltage measurement apparatus 300 erroneously reads the voltage left in the capacitor 3, and cannot detect the failure which may cause such an erroneous measurement.
According to one aspect of the present invention, a multiplex voltage measurement apparatus includes: (N+1) voltage detection terminals connected to N serially connected voltage sources; a capacitor which is charged with a voltage value of any of the N voltage sources; a first sample switch for selectively connecting odd-numbered voltage detection terminals among the (N+1) voltage detection terminals to a first terminal of the capacitor; a second sample switch for selectively connecting even-numbered voltage detection terminals among the (N+1) voltage detection terminals to a second terminal of the capacitor; a voltage measurement circuit for measuring the voltage value stored in the capacitor; a third sample switch for connecting the first terminal and the second terminal of the capacitor to the voltage measurement circuit; and a polarity controller for controlling the first and second sample switches such that one of the N voltage sources is selected while the third sample switch is open.
In one embodiment of the present invention, the polarity controller allows the first and second sample switches to sequentially select among the N voltage sources in a one-by-one manner such that the capacitor is alternately charged with voltage values having opposite polarities.
According to the present invention, when the voltages of voltage sources are sequentially measured, voltages having opposite polarities, i.e., positive voltages and negative voltages, are alternately applied to a capacitor. With such an arrangement, even when a first sample switch or a second sample switch is out of order so that it cannot be closed, and the voltage measured at a previous measurement remains in the capacitor, the voltage measurement apparatus can determine that there is a broken switch because a voltage measurement apparatus obtains voltage values of the same polarity in succession.
Thus, the present invention is characterized in that a voltage measurement apparatus includes a polarity controller for allowing the sample switches to sequentially select the voltage sources such that voltages having opposite polarities are alternately stored in the capacitor.
Thus, the invention described herein makes possible the advantages of (1) providing a multiplex voltage measurement apparatus which does not measure an erroneous voltage even when one of sample switches is out of order so that it cannot be closed; and (2) providing a multiplex voltage measurement apparatus which can detect a failure in an operation of one of the sample switches.